Input Order Robustness: Definition, Verification Procedure, and Examples

Verifying that logic control satisfies some desirable properties is essential to the proper and safe functioning of a manufacturing system. Input order robustness is one such property that has not been thoroughly explored. If a logic controller is input order robust for all sets of inputs whose elements can arrive in any order and whose order should not affect the logic controller´s final state or set of outputs, then the logic controller behaves the same in these regards irrespective of the inputs´ order. This paper develops a procedure to verify input order robustness for logic controllers implemented in a variety of formalisms, and demonstrates its application on Event-Condition-Action Modular Finite-State Machines (ECA MFSM) and IEC 61499 controllers. Additionally, this verification is extended to a class of networks of controllers, and the computational complexity of such verifications are discussed.